The present invention relates to an on-vehicle disk brake lathe which can provide an indication of the depth of cut on the surfaces of the disk, determine the lateral runout of the disk, and monitor the thickness of the disk which will result when the disk is machined.
There are a variety of measuring devices that are currently used in combination with bench lathes that allow monitoring of the surface being cut or, alternatively, measuring the position of the cutting tip. U.S. Pat. No. 5,765,457 provides an example of a bench lathe which provides a measuring system for determining the position of one or more cutting tips relative to a reference position that allows one to back out the depth of cut. The ""457 patent also teaches that, when the positions of both cutting tips are measured relative to a reference plane, one can calculate the thickness of the disk and determine a priori whether the disk is sufficiently thick that it can be machined to bring it into specification.
While the teaching of the ""457 patent offers a solution for machining of brake disks on a bench lathe, there is no teaching of the use of such a mechanism on an on-vehicle lathe. Furthermore, the system requires the use of two sensors and a complex algorithm to ascertain the position of the cutting tips, and from this information supplemental calculations are needed to determine the thickness of the disk.
Thus, there is a need for an on-vehicle brake lathe which employs rudimentary hardware to determine both depth of cut and whether a disk brake can be machined to bring it within the specifications of the manufacturer.
It is an object of the invention to provide an on-vehicle disk brake lathe that will determine the thickness of a resurfaced brake before the disk brake is resurfaced.
It is another object of the invention to determine the thickness of the disk brake using a single displacement gauge.
It is another object of the invention to provide a compact on-vehicle disk brake lathe which can be readily adjusted.
It is yet another object of the invention to provide a cutting tool assembly which can be retrofitted to an on-vehicle disk brake lathe for determining the depth of cut and disk thickness.
It is a further object of the invention to provide a method for determining the depth of cut of a tool bit for an on-vehicle disk brake lathe which employs a single displacement gauge.
It is still a further object of the invention to develop a method for resurfacing brake disks which allows an operator to qualify the brake disk for resurfacing before such is undertaken.
It is a further object of the invention to develop an automated brake disk machining system which reduces the likelihood of operator error.
The present invention relates to on-vehicle disk brake maintenance, and more particularly to an improved on-vehicle disk brake lathe of the type which is mounted to a wheel hub which, along with a brake disk, rotates about a hub axis; related methods for the machining to resurface disk brakes while they remain on the vehicle; and a system suitable for practicing the method of the present invention and automating the on-vehicle disk brake lathes of the present invention.
The on-vehicle disk brake lathe of the present invention relates to an improved cutting tool assembly for holding a first tool bit for resurfacing a first disk surface of the disk and a second tool bit for resurfacing a second disk surface of the disk. The cutting tool assembly can be either an integral part of an on-vehicle disk brake lathe or can be retrofitted to an existing on-vehicle disk brake lathe. In either case, the disk brake lathe has a lathe frame attachable to the wheel hub, a means for aligning a lathe axis with the hub axis, a drive mechanism for rotating the wheel hub and the brake disk about the lathe axis, and means for advancing the tool bits along a feed path which is normal to the lathe axis.
The improved cutting tool assembly for the on-vehicle disk brake lathe described above includes a first tool holder to which the first tool bit is mounted. The first tool holder is translatably engaged with respect to the lathe frame such that it traverses a first tool holder path which is parallel to the lathe axis. Means for moving the first tool holder along the first tool holder path are provided to allow adjusting the spatial separation between the first tool bit and the first disk surface.
A second tool holder is provided to which the second tool bit is mounted. The second tool holder is translatably engaged with respect to the lathe frame such that it traverses a second tool holder path which is parallel to the lathe axis, and means for moving the second tool holder along the second tool holder path so as to adjust the spatial separation between the second tool bit and the second disk surface are provided.
The means for moving the first tool holder and the second tool holder may be any of a variety of mechanical and electromechanical activating devices such as screw mechanisms, stepping motors, servos, rack-and-pinion mechanisms, and hydraulic or pneumatic actuators.
A displacement gauge is positioned relative to the first tool holder and the second tool holder so as to respond to changes in the spatial separation therebetween and to provide an output signal proportional to the separation. Gauges which are capable of providing such a response are further described in U.S. Pat. No. 5,970,427, assigned to the assignee of the present application.
Means for reporting the output signal of the displacement gauge are provided, which can typically be a display, printer, memory device or microprocessor. Frequently, these means are an integral part of commercially available gauges. When a microprocessor is employed, the signal provided to the microprocessor should be a digital signal, which can be either the inherent output of the displacement gauge or, alternatively, can be provided by an A/D converter.
It is further preferred that means for coordinating the motion of the tool holders be provided. Such means can be electronic or mechanical in nature, or can be a combination of electrical circuits and mechanical devices.
It is also preferred for the improved cutting tool assembly to have means for providing pair-wise displacement of the tool holders with respect to the brake disk. One preferred means for providing pair-wise movement of the tool holders, which is partly mechanical, is to employ a support plate which is translatably attached with respect to a platform and traverses a path that is parallel to the lathe axis, while the platform moves along the feed path normal to the lathe axis. The support plate in turn supports the tool holders, of which at least one is slidably engaged therewith such that its tool holder path is parallel to the path of the support plate.
When a support plate is employed, means for positioning the support plate relative to the disk are provided. In an elementary manner, this might be done by hand; however, mechanical means are preferred. A jack screw which is turned by a crank handle is an example of one such means which could be employed. Alternatively, a rack and pinion could be substituted for the jack screw, and the crank handle which is turned by the operator could be replaced with a servo mechanism.
When means for positioning the tool holders do not provide sufficient inherent structure to assure that the tool holders remain axially fixed with respect to the lathe frame as the tool bits traverse the disk surfaces, supplemental means for maintaining each of the tool holders in a fixed axial position relative to the frame are preferably provided to hold the tool holders in position while the tool bits are advanced across the disk brake surface being machined.
While the above described cutting head assembly and on-vehicle brake lathe is capable of determining the thickness of a machined disk brake, the displacement gauge must be calibrated to provide measurements of thickness. This can be done by having the operator bring the two tool bits into contact with each other prior to placing the tool bits around the brake disk. The operator notes the displacement gauge reading at this point, thereafter separates the cutting tips, and obtains the separation by subtracting out the reference reading. This procedure can be automated by a variety of techniques. For example, the tool bits could have an electrical potential maintained therebetween and the tool holders advanced until a current is sensed, thereupon the advancement being terminated and the displacement gauge zeroed. Similarly, rather than bringing the tool bits into contact, they could be brought to a standard separation and thereafter this reference value used as the reference number for determining the separation of the tool bits. While such calibration can be done manually, it is preferred to have an automated calibration mechanism to reduce the dependance on operator skills.
The present invention also provides a method for using an on-car disk brake lathe to machine a brake disk having a first disk surface and a second disk surface while the disk is mounted to a wheel hub which in turn is rotatably mounted about a hub axis. When implemented with the improved brake lathe of the present invention, this method requires only a single displacement gauge for monitoring the lathe operation and reduces the training required of the operator. To practice the method, the on-car disk brake lathe, which has a lathe axis and a lathe frame, is affixed with respect to the hub and the lathe axis is aligned with the hub axis. The wheel hub is then driven by the lathe to rotate the disk about the hub axis.
As the disk is rotated, the tool holders of the lathe are radially positioned with respect to the brake disk such that the tool bits are preferably positioned over the outer extremity of the wear surfaces of the disk. To determine lateral run-out, the tool bits should be positioned near the maximum diameter of the disk surfaces; however, when the brake disk has been in service, the outermost region of the disk is frequently not subject to wear by brake pads, which results in a ridge. This ridge is nominally the outer ⅙xe2x80x3 to xe2x85x9xe2x80x3 of the diameter of the disk. This region is also subject to corrosion, and thus is preferably machined away before such time as the thickness of the disk is measured. Thus, this ridged region of the disk should not be employed to determine the lateral runout or the thickness of the disk.
When the tool bits are positioned over the outer diameter of the wear surfaces, the first tool holder is moved to advance the first tool bit towards the first disk surface until the first tool bit makes continuous cutting contact with the first disk surface. Continuous contact can be directly observed by the operator by watching for the formation of a continuous machining chip, by listening for continuous sound of engagement of the tool bit with the disk, or by watching for the cutting of a continuous track. Similarly, the second tool holder is adjusted as described above with respect to the first tool holder to bring the second tool bit into continuous cutting contact with the second disk surface. When both tool bits are in continuous contact with their respective disk surfaces, advancement of the tool holders is ceased and the separation of the tool bits is measured. This measurement corresponds to the thickness of the brake disk which will result from machining. Such measurement can be readily made when the improved on-vehicle lathe of the present application is employed, since this separation will be directly obtainable from the displacement gauge.
If the operator has access to the minimum acceptable thickness as specified by the manufacturer, the measured thickness is preferably compared to the specified thickness. Only if the measured thickness is at least as great as the minimum acceptable thickness are the surfaces of the disk resurfaced by the tool bits. If the separation of the tool bits is less than the acceptable minium value, then the disk is discarded without further machining.
Even when the tool bits are positioned near the maximum radius of the disk surfaces, it may be advisable to increase each of the cuts taken by the tool bits by a predetermined amount below the depth that provides a continuous cut for the single ring path, both to assure that the lateral runout of the disk is corrected and to provide a smooth surface and reduce tool bit wear due to intermittent contact with the disk surface during machining. For a disk brake lathe employing the improved cutting head assembly which employs a single displacement gauge, this can be accomplished by monitoring the relative position of the first tool bit relative to the second tool bit and advancing one with respect to the other as one remains stationary. The amount of advancement for increasing the depth of cut equals the change in separation. To increase the depth of cut for the remaining tool bit, the new separation is recorded and thereafter this tool bit can then be advanced while the previously moved tool bit remains stationary. While such a procedure can be done, the predetermined cuts must be maintained sufficiently shallow as to avoid machining to a thickness less than the minimal acceptable thickness. This requires an additional check by the operator and adds one more step to the operator""s processing of the brake disk.
The practice of the method set forth above, even when employing the improved brake disk lathe of the present application, is at best laborious and at worst subject to operator error. Making such a determination of whether to machine the disk surface manually requires a multi-step procedure where the operator must obtain the specification of thickness which is appropriate for the brake disk being machined, which requires either obtaining the specification data from a manual or some other database. The operator must also compare this value against a separation measured when both tools have been advanced to the position where they are both in continuous contact with their respective disk surfaces as the disk is rotated. The success of these steps is dependent on the skill of the operator and the attention given to executing these steps. Thus, it is preferred to provide an automated system for determining, prior to machining, whether the resulting thickness of the brake disk will meet the specification. For this reason, it is preferable to employ a system that shares the operational duties between the lathe, the lathe operator, a microprocessor, and control circuitry.
The system of the present invention has an on-vehicle brake lathe having a disk cutting assembly which includes tool holders which support disk-engaging tool bits. Means for monitoring the separation between the tool bits are also provided, preferably by a displacement gauge responsive to the separation between the tool holders. An operator control panel is provided which contains hardware and circuitry to allow the operator to position the tool bits with respect to the brake disk. A microprocessor communicating with the means for monitoring the separation between the tool bits is provided.
In an automated system, it is preferred to have at least one contact sensor, which is responsive to contact between at least one of the tool bits and another object, such as the other tool bit or the brake disk. At least one set of instructions responsive to the at least one contact sensor is provided to direct the position of the tool bits with respect to the brake disk. An operator interface is provided allowing the operator to input data and select instructions which regulate the functions performed by the at least one set of instructions.
The microprocessor is also preferably provided with an input/output interface which allows it to communicate either directly or indirectly with the operator. The input/output interface can also be employed to input standard specifications for the brake disk. These standard specifications for the disk brake as specified by the manufacturer can be read into the microprocessor in response to prompts from the operator interface. These specifications can be provided to the microprocessor by a variety of data transfer techniques, such as inputting the data via floppy disk, CD ROM, zip disk, on-line transfer keyboard, etc. Further discussion of means for inputting data is contained in the ""427 patent noted above. The microprocessor can then be employed for comparing the tool bit separation, when both tool bits are in continuous cutting contact with the brake disk to the minimum standards which are provided by using an appropriate instruction set. This can be done before the tool bits are advanced across the disk surfaces. The microprocessor can also generate a comparison output signal. Depending on the output signal, a display is generated which indicates to the operator whether the tool bit separation falls within the brake standard specifications, at which point the operator is signaled to advance the tool bits across the disk surfaces to resurface the disk. Otherwise, the operator is signaled that the disk should be discarded, thereby eliminating any unnecessary machining of the disk to save operator time as well as wear on the tool bits.
It is further preferred that the means for monitoring the separation between the tool bits be provided by a displacement gauge which monitors the separation of the tool holders. When it such is the situation, the displacement gauge must be calibrated. To calibrate the gauge, the microprocessor can be provided with a series of instructions which result in the tool bits being brought into contact to determine a zero separation of the tool bits, at which point the advancement of the tool bits is stopped and the gauge output signal is zeroed or a base value established, and thereafter the tool bits are separated with the gauge reading the separation of the tool bits or the separation of the tool bits plus the base value. Alternative schemes can be employed, where the calibration is done by providing stops which are positioned such that, when engaged by the tool holders, the tool holders will be at a known separation from which the separation of the tool bits can be determined. In either case, these instructions can be manually implemented by the operator or, when the lathe is designed to be responsive to the microprocessor, these instructions can be performed under the direction of the microprocessor. In the latter case, one or more contact sensors are preferably employed to provide signals to indicate to the microprocessor when the tool bits are in contact with each other or are in either initial or continuous cutting contact with the disk surfaces.
When the system is designed to automatically determine when the tool bits are in continuous contact with the disk, the at least one contact sensor is a contact gauge which is capable of distinguishing between intermittent and continuous contact. Such gauges would include vibration, acoustic, and optical gauges. When the tool bits are advanced individually, a single contact gauge can be used sequentially for each tool bit as it is advanced to determine when initial contact and continuous cutting contact have been achieved. However, for simplicity of the instrumentation to distinguish between continuous contact for the separate tool bits, it is preferred to provide a first contact gauge to monitor the first tool bit and a second contact gauge to monitor the second tool bit.
When it is further desired for the system to have the capacity to measure the lateral run-out of the disk without contacting the disk with the tool bits, such can be done by providing a surface displacement gauge positioned such that it will be responsive to variation in the distance to the disk. Preferably, the surface displacement gauge is mounted to one of the tool holders so as to engage the disk at approximately the same radius as the tool bits, and monitors the separation between the tool holder and the disk surface when the holder is fixed in position. Circuitry is provided such that the surface displacement gauge reports distances only when the gauge is stationary. In this way, the lateral run-out can be determined from separation data provided by the surface displacement gauge. To assure that the surface displacement gauge is within its range, it is further preferred for a proximity sensor to be provided to verify that the position of the tool holder is such that the distance from the surface displacement gauge to the disk surface being measured remains within the range of the gauge as the disk is rotated.
The above described system can be programmed to provide alternate functions. For example, the above described system can be programmed such that, when an independent determination has previously been made of the thickness and lateral runout of the disk, an alternative method for machining the brake disk may be employed. This is frequently the case when the disk has been previously tested to determine whether machining is necessary. When such is the situation, the tool bits need not be brought into continuous cutting contact with the disk after mounting the lathe and positioning the tool bits about the disk. However, at least one reference point must be established, which can be done by bringing one of the tool bits into initial contact with the disk as the wheel hub and disk are rotated. With this reference point established and with the lateral runout information obtained from prior testing, it is determined how deep the cut on each surface of the disk will need to be to correct such lateral run-out. The sum of the depth of both cuts can be subtracted from the sum of the previously determined thickness and the lateral runout to provide a calculated resulting thickness, and this value can be compared to the manufacturer""s specification. If the calculated resulting thickness is sufficient, then the tool bits can be positioned with respect to the reference point to set the desired depths of cut and thereafter advanced across the disk surfaces to remove the sufficient material to bring the brake disk into specification.
When it is desired to have the lathe resurface the disk surfaces from the outer region of the disk surfaces to the interior region, a proximity sensor can be employed to assure that the inward advance of the tool bits is stopped prior to contact with the wheel hub of the vehicle. Additional proximity sensors can be employed which can prevent accidental collision between the tool holders or tool bits and the disk or other fixed parts of the vehicle as the tool bits are positioned. Such additional proximity sensors can be particularly important when the system is automated and thus not subject to intervention by the operator.